The invention pertains to current sensors and particularly to fiber optic current sensors. More particularly, the invention pertains to fiber optic current sensors having improved isolation.
Fiber optic current sensors operate on the principle that the magnetic field produced by an electrical current affects certain properties of the light contained in an optical fiber wound around the current carrying conductor. Through the Faraday effect, those properties affected can be either the polarization state of the light (polarimetric type sensor) or the velocity of the light (interferometric type sensor). Through Ampere""s law,
Hxc2x7dl=I,xe2x80x83xe2x80x83(1)
it is evident that for the current sensor to make an accurate determination of the current, I, the light in the fiber should be uniformly and linearly sensitive to the magnetic field, H, and the sensitive region should comprise as perfectly a closed path as possible. In this case, the sensor substantially measures Hxc2x7dl, thereby giving an indication of I as an output, provided that the sensor is well isolated against currents flowing outside the sensing loop. In addition, the sensor should return the correct value of I regardless of the actual location of the current flowing through the sensing coil.
A number of applications for current sensing exist which require the sensor to exhibit an extremely good isolation from external currents as well as extremely uniform response to currents that pass through the sensing coil at different physical locations. For example, a ground fault interrupter for large currents may have a difference current measurement system 11 with a sensor coil or head 14 that encloses both the outgoing 12 and return 13 currents (FIG. 5). Hundreds of amperes of current may flow through the wires, while a difference between the two currents 12 and 13 of a few milliamperes should be quickly recognized. Such a system may exist in the vicinity of many other conductors carrying hundreds of amperes of current. The isolation of sensor head 14 to external currents should therefore be better than ten parts-per-million, and sensor system 11 should respond uniformly to the outgoing and return currents to within ten parts-per-million.
A second example of how a fiber optic current sensor may advantageously benefit from good isolation/uniformity performance is the construction of a fiber optic current sensor 15 assisted current transformer 16 (FIG. 7). In this device, fiber optic current sensor 15 is operated using a secondary current 19 from current supply 49 to null the output (i.e., close the loop). A current 18 to be measured passes through a sensing coil or head 17, while an equal and opposite loop closing current 19 passes through the sensing coil 16, possibly through multiple turns. Loop closing current 19 includes the secondary of this fiber optic current sensor 15 assisted current transformer. The accuracy of this device depends on current sensor 15 exhibiting uniform response to currents passing therethrough for all the different physical locations of current 18.
A third example of a fiber optic current sensor requiring superior isolation is the displacement current based voltage sensor 20 (FIG. 6). In this device, an AC voltage 21 is measured by integrating (by integrator 36 via electro-optics module 37) the output of a current sensor head 22 that responds to displacement current. Typically, sensor 20 might measure a few milliamperes of displacement current. The power line, which carries voltage 21 to be measured, may also carry a real current, which might typically be a few thousand amperes. Thus, to obtain a true measure of the voltage, it is necessary for the current sensor head 22 to be well isolated from the real current flowing through the power line. The isolation requirement for this application may easily exceed one part-per-million.
A problem with Faraday effect based optical current sensors, both polarimetric 23 (FIG. 2) and interferometric 24, 25 (FIGS. 3 and 4), is that the sensitivity of the light to the local magnetic field depends on the exact polarization state of the light at that point. It is very difficult to maintain a strictly uniform state of polarization of the light throughout a sensing path of the sensing head or coil, as stresses within the glass induce local birefringences that alter the polarization state of the light. Thus, a method of desensitizing the sensor head to these imperfections is needed in order to achieve the overall intended isolation and uniformity requirements.
It has been discovered that maintaining an unaltered polarization state of the light throughout the sensing loop(s) is not a practical necessity to achieve superior isolation and uniformity performance of the sensor. Rather, a sufficient requirement on the sensor head or coil for achieving good isolation and uniformity is that it not exhibit long period undulations in sensitivity. Undulations having long periods reduce isolation of the sensor head so as to be sensitive to other currents not intended to be measured. Accordingly, set forth here are design approaches for fiber optic current sensors that reduce long period undulations in the sensitivity of the sensing head coil. Remaining rapid undulations contribute negligibly to uniformity and isolation errors.